In this specification, the term "flow meter" refers to a flow velocity and flow rate measuring instrument, keeping in mind that in the case of measurements based on the propagation of acoustical waves flow rate is a function of flow velocity.
The growing need for highly accurate measurements of flow rates of all kinds of fluids, including petroleum products, accounts for increasingly stringent requirements imposed today on pulse-frequency ultrasonic flow rate measuring techniques carried out with the aid of ultrasonic flow meters of the type that comprises a synchronized ring circuit, i.e. a pulse generating system with a delayed acoustic feedback. Flow meters of this type are accurate only if the triggering results in a continuous autocirculation of a pulse through the synchronized ring circuit. This statement applies only to the pulse that carries useful information. In actual measuring systems, however, the acoustical channel, i.e. the spacing intended for passage of a medium to undergo flow rate measurements and found between two electroacoustical transducers, is subject to periodic disturbances which affect the accuracy of measurements and are due to the scattering of the ultrasonic beam by gas bubbles and foreign particles contained in the medium. With foreign particles drawn away by the flow, one must restore the operating state of the flow meter, i.e. the continuous autocirculation of a pulse through the synchronizing ring circuit. A delayed triggering of the flow meter or an autocirculation of two or more pulses through the ring circuit distort the results of the measurements.
There is known a method for triggering a pulse-frequency ultrasonic flow meter, consisting of simultaneously turning on the synchronizing ring circuit and applying a trigger pulse thereto.
There is known an ultrasonic flow meter adapted for this method and comprising two synchronized ring circuits, each incorporating, in a series arrangement, an amplifier, an excitation pulse former and two electroacoustical transducers separated by a gap intended for passage of a medium to be subjected to flow rate measurements. The electroacoustical transducers are so oriented in relation to each other that one of them can transmit and the other receive an acoustical signal traveling at an angle .alpha. other than 90.degree. to the flow direction. The flow meter under consideration further includes an auxiliary oscillator and a measuring unit which are both connected to the synchronized ring circuits.
In each synchronized ring circuit of such a flow meter, the oscillator transmits a pulse to the excitation pulse former whose output signal is applied to that electroacoustical transducer which sends a pulse into the medium. This pulse is received by the second electroacoustical transducer and again applied to the excitation pulse former, whereby an autocirculation of pulses in the synchronized ring circuit is effected. The difference .DELTA.f of the frequencies at which the autocirculation of pulses takes place in the synchronized ring circuits is indicative of the flow velocity V.
For example, with two electroacoustical transducers arranged on the opposite sides of a pipeline whose diameter is D, EQU .DELTA.f=Sin2.alpha../D V (1)
However, the method under review does not provide for an automatic retriggering of the flow meter following a temporary disturbance in the acoustic channel, because it does not include the operation of resuming the autocirculation of a pulse through the synchronized ring circuit. The autocirculation indicates that the flow meter is in good working order and unless it takes place, one must bring into play and auxiliary oscillator so as to apply a trigger pulse to the synchronized ring circuit.
The above disadvantage is eliminated in another method for triggering a pulse-frequency ultrasonic flow meter. The method is as follows. At the start of the triggering, the pulse former of the synchronized ring circuit operates in the self-oscillation mode, its natural oscillation period being selected to be somewhat greater than the maximum pulse propagation time in the synchronized ring circuit. As in the foregoing case, the former transmits a pulse to an emitting transducer whose signal is received by the receiving transducer. This signal is received before the former of the synchronized ring circuit produces a second pulse. The received pulse accounts for a forced triggering of the former, whereby the latter operates in the forced oscillation mode.
If there are disturbances in the acoustic channel, no pulses arrive from the receiving transducer. The presence of detected voltage indicates that the flow meter is back in its operating state and that one may record the measurements.
There is known an ultrasonic flow meter adapted for the aforedescribed case method and comprising two synchronized ring circuits connected to a measuring unit and an amplitude discriminator. The trigger pulse former of each synchronized ring circuit can operate in both the self-oscillation and single-shot modes. At the instant the flow meter is triggered into action, the pulse former of the synchronized ring circuit is operating in the self-oscillation mode, its natural oscillation period being selected to be somewhat greater than the maximum pulse propagation time in the synchronized ring circuit. Similarly to the aforedescribed case, the former transmits a pulse to the emitting transducer, which is received by the receiving transducer. The reception takes place before the former of the synchronized ring circuit produces a second pulse. The received pulse brings about a forced triggering of the former which operates in the forced oscillation mode. If there are disturbances in the acoustic channel, no pulses arrive from the receiving transducer, and the former operates in the self-oscillation mode. As soon as the normal working condition of the acoustic channel is re-established, the first received pulse brings about a forced triggering of the former of the synchronized ring circuit, whereby the flow meter is brought back to its operating state. In order to ascertain the correctness of measurements, the amplitude detection of the receiving transducer's output signal is carried out, and the output voltage of the amplitude detector indicates that the flow meter is in the working state.
The foregoing method is disadvantageous in that the operating condition of the flow meter cannot be assessed correctly in the presence of interference at its input.
All the aforedescribed types of flow meters have a low noise immunity both in the course of triggering and during operation. This is due to the fact that the synchronized ring circuit conducts current throughout the operation. If the former of the synchronized ring circuit is actuated by a spurious signal, two or more signals may circulate through the ring circuit, distorting the measurements.
Also known is a method for triggering an ultrasonic flow meter comprising a synchronized ring circuit, which method consists in applying trigger pulses to the synchronized ring circuit, whereby the latter is periodically turned on and off. According to the method, the information on the presence of a pulse circulating through the synchronized ring circuit is stored so as to restore the operating condition of the flow meter following a temporary disturbance in the acoustic channel. The noise immunity of the flow meter is improved by turning the synchronized ring circuit off for a period of time which is shorter than the estimated time of propagation of a signal in the electroacoustic channel.
A known ultrasonic flow meter is adapted for the foregoing triggering technique and comprises at least one synchronized ring circuit composed, in a series arrangement, of a shaping amplifier, an inhibitor, an excitation pulse former and two electroacoustical converters separated by a gap intended for passage of a medium to be subjected to flow rate measurements. The transducers are oriented with respect to each other so as to enable one of them to transmit and the other receive an acoustical signal traveling at an angle other than 90.degree. to the direction of the flow. The flow meter further includes a trigger pulse unit and a measuring unit which are connected to the synchronized ring circuit. Finally, the flow meter incorporates a one-shot oscillator connected to the inhibitor. The trigger pulse unit is a delayed feedback oscillator (cf. USSR Inventor's Certificate No. 526,827, C1. G01 P 5/00).
The latter flow meter operates as follows. The autocirculation pulse actuates the delayed feedback oscillator which triggers the flow meter back into action following a temporary disturbance in the acoustic channel. In order to raise the noise immunity of the flow meter, the synchronized ring circuit is turned on by the one-shot oscillator which is actuated by a pulse received by the electroacoustical transducer. This oscillator is actuated for a period of time which is shorter than the estimated time of propagation of the signal in the electroacoustical channel.
The aforedescribed method is disadvantageous in that it does not provide for automatically triggering the flow meter. Besides, an increase of the time of propagation of the signal in the medium accounts for a prolongation of the period during which the synchronized ring circuit conducts current, which means there is a possibility of the synchronized ring circuit being turned on by a spurious signal.
The flow meter under consideration cannot be triggered automatically, which is an important drawback if it is to be incorporated in an automatic flow rate control system. An increased propagation time of the signal in the acoustic channel means a longer time during which the synchronized ring circuit conducts current. The resultant possibility of the synchronized ring circuit being brought into action by a spurious signal affects the accuracy of measurements.
Besides, with low flow velocities, the difference of the frequencies in Equation (1) is quite small. Consider this example: .alpha.=45.degree., D=1 m, and V=0.1 m/sec; in this case, .DELTA.f=0.1 Hz, which means that the measurement time is 10 seconds; clearly, this is too long.
The above considerations rule out the possibility of measuring instantaneous flow rates, which also affects the overall accuracy of measurements. On the whole, the flow meter under consideration does not fit into automatic control systems.